It is known to use ultrasound to carry our location determination, e.g., on a working surface such as a whiteboard or a projection screen. I call such systems pen-stroke annotation capture systems. The term stylus is used in this description to refer to a movable pointing device that includes a transmitter of ultrasound and of electromagnetic signals, e.g., infrared (IR) signals. A capture system placed near the working surface in which location is to be determined includes ultrasound receivers for receiving and sensing the transmitted ultrasound and a sensor for receiving infrared signals. The infrared signals provide a time reference. Such a stylus may be marking, e.g., a pen or retrofitted pen, or a non-marking pointing device used for interaction, or an erasing device. Various methods can be used to take into account the difference in time of arrival of the ultrasound transmitted from the stylus to the two (or more) ultrasonic receivers to determine the location of the stylus/pen/pointer in the working surface. Such a capture system typically includes an interface, e.g., a USB connection or wireless connection, to send information to a computer. Such a system, in combination with the computer, captures the pen strokes on the computer, including the color of the pen in the case of a stylus that marks, and any erasing.
Recently, there has been widespread use of a location determining system with a projector that projects onto a screen area that becomes an active area for the stylus. Thus, by adding an electronic pen-stroke annotation capture system (in general, what I call a “capture system”) that is connected to the computer, any flat surface, e.g., a whiteboard used as a projection screen area, may be converted into an area wherein pen strokes are capturable to the computer. Thus, one or more computer-generated images may be projected on the projection screen surface, and pen strokes may be drawn by the stylus over the computer generated image. Such pen strokes are then capturable by the location system and transferable to the computer. The pen strokes drawn over the projected image may be captured in the computer for later playback and analysis, including synchronized playback with the projected image or images.
Recently there has been a desire to have the ultrasound receivers and IR sensor not on the working surface, but rather some distance away from the surface. For example, there are several projectors that are placed away from, but still relatively close to, the working surface, which in the case of a projector, is a screen area. For example, NEC Corporation of Tokyo Japan makes a series of projectors, such as model WT610 and model WT615. Each such NEC projector is able to project images with an image diagonal of 40 inches at a surface distance of only 5½ in, accomplished with the help of an innovative highly complex mirroring system. 3M and others also make systems that are able to project an image from a short distance away from the screen. With such systems, it is possible and desirable to incorporate the ultrasound receivers and infrared receiver, as well as the location determining electronics, e.g., the ultrasound receivers and related electronics, into the projector itself. In such a case, the receivers are at a slightly different plane than the working surface, which is the screen
FIG. 1 shows, in simplified and exaggerated form, a projector on the floor some relatively short distance from a screen which forms a working surface. The projector includes a sensor array including ultrasound sensors at known locations and an infrared sensor coupled to electronics and processing to determine the location of a stylus such as a pen or pointing device or erasing device that includes an ultrasound transmitter including an ultrasound transducer, and an infrared transmitter. FIG. 1 shows the tip of the pointing device in hugely magnified form.
It is desired, e.g., in the case of projecting, but also in other situations such as in the case of a whiteboard, or in the case of a flat panel display such as an LCD display or a plasma display, to have the ultrasound transmitter not be at the same plane as the working surface. There are several challenges for ultrasound based position capturing systems when the ultrasound transmitter is offset from the working surface and the ultrasound receiver is not located on the same plane as the working surface. The most serious of these issues include reflection off the working surface, stylus tip offset, and a signal strength that is too low.
Reflection Off the Working Surface
If an ultrasound receiver is not placed on the working surface, when a stylus, e.g., a pen, or pointer is transmitting an ultrasound signal, there will be two main paths for the signal rather than one path as desired: a direct signal and a reflected signal off the working surface as shown in FIG. 1. Often the phase differences of the two signals range from less than one wavelength to a few wavelengths for a 40 KHz ultrasound. The receiver picks up both the direct and the reflected signals, and when the difference is odd multiples of half wave length (2n−1)π, n=1, 2, 3 . . . , the signals will tend to attenuate each other and greatly reduce the available signal with which to. The ultra sound receivers placed on the projector away from the working surface may receive different amount of interference between the direct and reflection waves, depending on the proximity and angle relative to the stylus, causing the combined wave shape differ significantly among receivers
Stylus Tip Offset
Differences in the angle at which the stylus is held are amplified by the tilt of the transducer from the working surface, which causes the location of the stylus tip, e.g., tip of the pen, eraser, or pointer, as resolved by the ultrasound receiver system, to change where there is different tilt.
Signal Strength is too Low
The received ultrasound signal strength is enhanced when the ultrasound waves propagate along a hard surface, e.g., along the working surface, and is attenuated when the ultrasound waves propagate directly through air. Furthermore, the signal strength will be reduced significantly when reflection of ultrasound from the working surface destructively adds into the direct signal at phase close to odd integer multiples of half of the wave length.
U.S. patent application Ser. No. 11/764,757 describes a method and a stylus. The stylus includes an acoustic transmitter at a first location and a wave directing mechanism configured to direct acoustic waves from the acoustic wave transmitter to a tip location near the end of the tip of the stylus. One version includes a waveguide. Another version includes a shaped reflector behind the acoustic transmitter shaped to focus acoustic waves from the acoustic wave transmitter to the tip location. Another version includes using an array of ultrasound transducers driven by electronics arranged to focus ultrasound energy towards the tip location. Yet another version uses an acoustic lens.
U.S. Application US 20040169439 to inventor Toda describes a stylus that includes a tip and an ultrasound transducer, either a cylindrical piezoelectric transducer having a holder and a cylindrical piezoelectric film spanning between at least two spaced apart cylindrical surfaces of a holder, or a flat transducer having a diaphragm, and a piezoelectric material disposed on a surface of the diaphragm. Toda includes a waveguide designed to focus energy on a tip.